Technical Field
This invention relates to a stent covering member for holding a stent, which is to be expanded in diameter when implanted into a vessel such as blood vessels to scaffold the vessel from inside, in contracted state and a stent apparatus using this stent covering member.
Background Art
Heretofore, when stenosis occurs in a blood vessel, such as coronary artery, percutaneous transluminal coronary angioplasty (PTCA) employing stenting is performed in which the stenosed portion is expanded by using a medical balloon catheter, and a cylindrical stent is implanted into this expanded portion for scaffold the vessel from inside so as to ensure the blood flow.
The inventor of the present invention has proposed a stent formed of a biodegradable polymer which, once implanted in a blood vessel of a living body, disappears in the living body after passage of a period of time in WO92/15342(PLT 1), WO00/13737(PLT 2) and WO2009/157164(PLT 3).
These stents are formed of a biodegradable polymer having shape memory property as a cylindrical body and shape memorized to an expanded size to scaffold the blood vessel from inside. These stents are, once implanted in a blood vessel, heated by body temperature to expand to the shape memorized outer diameter and keep the expanded state to scaffold blood vessels from inside. The stent of this kind is mounted on a catheter, inserted together with the catheter into a blood vessel and implanted in a lesion site in the vessel.
The stent expanded to a size to scaffold a blood vessel is contracted to a size having an outer diameter sufficiently smaller than the shape memorized size and mounted on the catheter in this contracted state.
It should be noted that, when the stent formed of the biodegradable polymer material is heated by body temperature and expanded to the shape memorized outer diameter, the diameter is gradually expanded over a sufficient period of time rather than immediately due to the nature of the material. This is because the biodegradable polymer material has a property as a viscoelastic body and viscous resistance is produced when it expands to the shape memorized size. As explained hereinabove, the expansion of the stent needs a certain length of time.
The stent formed by using a biodegradable polymer material is delivered to an implantation position within the blood vessel, that is a lesion site, and then immediately expanded in diameter so that it is expanded to a size to scaffold the inner wall of the blood vessel by utilizing inflation force of a balloon capable of being rapidly expanded with injection of an expansion media.
The stent formed by using a biodegradable polymer material to be expanded by the above described balloon inflation force is mounted, in a contracted state, on the balloon which has been mounted on a tip of a catheter in a folded state, and is transported to the implantation site together with this balloon. When transported to the desired implantation position in the blood vessel, the stent is implanted at the lesion site by supplying the expansion media into the balloon and rapidly expanding the stent to a size to scaffold the blood vessel from the inside. The once expanded stent formed by using a biodegradable polymer material keeps the shape memorized size by its self-expansion force even after the balloon is deflated by removal of the expansion medium, thereby scaffolding the implanted site from the inside to allow fluid path for blood in the blood vessel.
The stent formed by using a biodegradable polymer material having shape memory property and being shape-memorized to the expanded size to scaffold the blood vessel from the inside in an expanded state is heated by body temperature as it is inserted into the blood vessel, generating self-expansion force to recover the shape from the contracted state to expanded state. By the effect of this shape restoring function, the stent having crimped to the balloon in contracted state is expanded in diameter, thus making a gap between the stent and the balloon. For this reason, dislocation or disengagement might occur between the balloon and the stent mounted onto the balloon due to the friction force generated when the stent contacts with an inner wall of the blood vessel as it is inserted into the blood vessel. The stent disengaged from the balloon cannot be rapidly expanded in diameter with the balloon inflation force, making correct implantation to the intended site such as the lesion site impossible. The stent not disengaged from the balloon but just dislocated relative to the balloon may be subjected to the balloon inflation force unequally along its entire length, resulting in its unequal expansion along its entire length. The stent expanded unequally along its length cannot scaffold the vessel wall in the vessel as intended.
It is reported that, in PTCA, when stenting is employed, restenosis occurs in high probability. To prevent this restenosis, drug-eluting stents have been proposed e.g. in Japanese Unexamined Patent Application Publication No. 2008-253707(PLT4), in which a coating agent containing a drug having intimal hyperplasia inhibiting efficacy or antithrombotic efficacy is applied to the surface thereof so that the drug is released for a predetermined period of time. In this case, however, if a stent is disengaged from a balloon or dislocated, thus disabling a correct implantation of the stent at a desired indwelling position, the drug carried by this stent cannot be released at a desired drug administration site.
In order to solve the problem described above, the inventors of the present invention has proposed a stent delivery apparatus wherein a balloon catheter, having a balloon on which the stent is mounted, is inserted into a protective sheath (WO2004/103450:PLT 5). Furthermore, in this stent delivery apparatus, a holding member holds one end of the stent to prevent stent dislocation relative to the balloon, when the stent is extruded from the protective sheath to be expand in diameter.